1. Field of the Invention
This invention relates to a process for polymerization of monoalkenyl aromatic monomers and/or acrylic monomers at temperatures and residence times that produce low molecular weight homomacromers or comacromers having terminal unsaturation.
2. Related Background Art
Polymerization of ethylenically unsaturated monomers at moderate temperatures to produce low molecular weight polymers is known in the art. U.S. Pat. No. 4,529,787 describes a process for polymerizing mixtures of monoalkenyl aromatic monomers and acrylic monomers in the temperature range from 180xc2x0 C. to 270xc2x0 C., with a residence time from 2-60 minutes. The same temperature range is employed in the method described in U.S. Pat. No. 4,546,160 for polymerization of mixtures containing only acrylic monomers. In this reference, the residence time varies from 1-30 minutes. Both of temperatures above 270xc2x0 C. leads to adverse effects on the products, for example, discoloration, oxidation, depolymerization, and side reactions.
The methods of the aforementioned references employ free radical polymerization initiators. This is typical of most known methods for production of low molecular weight polymers. For example, monoethylenically unsaturated carboxylic acids, such as acrylic acid, may be converted to low molecular weight polymers in aqueous solution in the presence of an initiator, as described in U.S. Pat. No. 5,268,437. This method employs temperatures in the range from 130xc2x0 C. to 240xc2x0 C. Another method for production of low molecular weight acrylic and hydroxyl-containing polymers is described in U.S. Pat. No. 4,600,755. This method requires use of aqueous hydrogen peroxide as an initiator at temperatures above 140xc2x0 C. Copper salts are used as initiators for production of low molecular weight polymers of monoethylenically unsaturated carboxylic acids and esters in U.S. Pat. No. 5,100,980.
The production of low molecular weight oligomers using cobalt chain transfer agents is also well known. For example, U.S. Pat. No. 4,680,352 describes the use of Co(II) chain transfer agents to control the molecular weight of free radical polymerization of acrylic and/or styrene monomers. The preparation of macromers that are both copolymers and homopolymers using specific cobalt (II) chelates as catalytic chain transfer agents is described in U.S. Pat. No. 5,028,677. Suitable reaction temperatures are said to be between about 50xc2x0 C. and 150xc2x0 C. The preparation of terminally unsaturated monomers using such cobalt-containing chelate transfer agents is described in U.S. Pat. No. 5,587,431. The process temperatures are said to range from room temperature to 200xc2x0 C. or higher, but preferably between about 40xc2x0 C. and 100xc2x0 C. U.S. Pat. No. 5,324,879 describes the preparation of oligomers using cobalt (III) chain transfer agents and exemplifies reaction temperatures between 60xc2x0 C. and 95xc2x0 C.
Other types of chain transfer agents have also been employed. Japanese Patent Publication No. 62-232408 describes the preparation of an acrylic polymer having a functional group on one terminal by heating a mixture of an acrylic monomer, a functional group chain transfer agent and a polymerization initiator to about 75xc2x0 C. in an inert atmosphere followed by addition to a corresponding polymerization system to form a polymer having a number average molecular weight of 1000 to 30,000. Macromers prepared using xcfx89-unsaturated oligomer as a chain transfer agent are described in U.S. Pat. No. 5,362,826. Reaction temperatures of 20xc2x0 C. to 200xc2x0 C. are said to be suitable.
Other various techniques for preparing macromers have also been employed. Japanese Patent Publication No. 60-133007 describes the preparation of a macromer obtained by reacting a polymer containing a carboxyl group derived from mercaptopropionic acid at one end and a glycidyl group-containing monomer in the presence of a catalyst. Macromers prepared with a monomer unit having an xe2x80x94SO3M functional group, wherein M is alkali metal or ammonium, are described in Japanese Patent Publication No. 05-247117, Japanese Patent Publication No. 04-119526 and Japanese Patent Publication No. 05-287010. A macromer containing a carboxyl group in the main chain and a vinyl-polymerizable functional group solely on the chain terminal end is described in Japanese Patent Publication No. 01-268,709. U.S. Pat. No. 5,147,952 is directed to the preparation of macromers by anionic polymerization of methacrylic acid ester monomer using a specific polymerization initiator.
Thermally-initiated polymerization, in which a free-radical polymerization process is initiated by heating rather than by addition of initiators, has been used to prepare low molecular weight polymers from ethylenically unsaturated monomers. U.S. Pat. No. 4,414,370 describes a thermally-initiated polymerization process for preparing low molecular weight polymers in a continuous reactor, at temperatures from 235xc2x0 C. to 310xc2x0 C., with a residence time of at least 2 minutes. This reference suggests that polymerization at temperatures above 310xc2x0 C. leads to adverse effects on the products, for example, discoloration, oxidation, depolymerization, and side reactions. Copolymerization of monoalkenyl aromatic monomers with acrylic monomers are disclosed. However, polymerization of either type of monomer alone under these conditions is not exemplified.
Higher temperatures are employed in the polymerization process described in European Patent Application EP 687,690. This reference describes the use of a temperature range of 250xc2x0 C. to 500xc2x0 C. for polymerization of acrylic acid in the presence of at least one initiator, with optional addition of other acrylate monomers or monoalkenyl aromatic monomers, to produce low molecular weight polymers with terminal unsaturation, also referred to as macromers. However, the residence time is limited to the range from 0.1 seconds to 5 minutes. Polymerizations at these temperatures for longer residence times are said to be undesirable because the yield of terminally unsaturated oligomers will be decreased without substantially affecting molecular weight or monomer conversion to polymer.
A process for polymerization of acrylic monomers and/or monoalkenyl aromatic monomers at high temperatures or long residence times which provide low molecular weight homomacromers and polymacromers having terminal unsaturation would be highly desirable.
This invention is directed to a process for preparing low molecular weight macromers, i.e., homomacromers and comacromers, having terminal unsaturation.
In one embodiment, homomacromers are prepared by the process comprising the steps of (a) charging a reactor with (i) a single ethylenically unsaturated monomer selected from the group consisting of acrylic monomers or monoalkenyl aromatic monomers and (ii) if said ethylenically unsaturated monomer is not a thermally initiating monomer, a free radical polymerization initiator, to form a polymerizable reaction charge; and (b) heating the reaction charge during continuous mixing in the reactor to a reaction temperature in the range from about 180xc2x0 C. to about 500xc2x0 C. for a residence time greater than 5 minutes to form the low molecular weight homomacromers, provided that if the residence time is 60 minutes or less that the reaction temperature is greater than 270xc2x0 C.
Yet another embodiment is directed to the preparation of comacromers by (a) charging a reactor with (i) at least two ethylenically unsaturated monomers selected from the group consisting of acrylic monomers, monoalkenyl aromatic monomers or mixtures thereof and (ii) if at least one of said ethylenically unsaturated monomers is not a thermally initiating monomer, a free radical polymerization initiator, to form a polymerizable reaction charge; and (b) heating the reaction charge during continuous mixing in the reactor to a reaction temperature in the range from about 180xc2x0 C. to about 500xc2x0 C. for a residence time greater than 5 minutes to form the low molecular weight comacromers, provided that if the residence time is 60 minutes or less that the reaction temperature is greater than 270xc2x0 C. and further provided that if the reaction charge is a mixture of acrylic monomer and monoalkenyl monomer and the residence time is 60 minutes or less then the reaction temperature is greater than 310xc2x0 C. Preferably the reaction temperature of the process of this invention is greater than 270xc2x0 C., more preferably greater than 310xc2x0 C.
The macromers prepared by the process of this invention, are terminally unsaturated, and thus are useful, for example, as monomers in additional polymerization reactions. The polymers that result from such additional polymerization reactions are useful in adhesives, coating compositions and the like.